The proposed experiments will address a fundamental hypothesis concerning the long-term consequences of stroke-induced neuronal injury. The hypothesis is that a hypoxic-ischemic insult leads to synaptic reorganization and increased seizure susceptibility at both the site of injury and other brain regions over the weeks and months following the injury. Using a juvenile-rat model of hypoxia-ischemic, we will determine the physiological consequences of long-term synaptic reorganization in the hippocampus that follow a hypoxic-ischemic insult. We will test the hypothesis that the sprouted mossy fibers produce new recurrent excitatory synapses. The strength of recurrent excitation will be assessed with whole-cell patch-clamp techniques combined with focal stimulation using flash photolysis of caged glutamate. Spontaneous and glutamate- evoked synaptic currents will be analyzed at different times after the hypoxic-ischemic injury. Alterations in the frequency and amplitude of excitatory postsynaptic currents (EPSCs) will be used to assess synaptic reorganization. Recorded neurons will be filled with biocytin and their morphology correlated to the physiological findings. The Timm stain histological procedure will be used to reveal the extent of axonal sprouting in the inner molecular layer of the dentate gyrus of adjacent slices. These proposed experiments will combine several techniques at the cellular level to examine the reorganization of local synaptic circuits after a hypoxic- ischemic injury. This research should indicate how stroke-related brain injury alters local synaptic circuits in a model system, which may contribute to cognitive deficits and epilepsy.